This invention relates to internal combustion engines in general and, more particularly, to methods and apparatus for reducing oxides of nitrogen in the exhaust of internal combustion engines.
The exhaust gas from an internal combustion engine includes a number of undesirable compounds, such as carbon monoxide, hydrocarbons, and oxides of nitrogen (NOx), namely nitrogen dioxide (NO2) and nitric oxide (NO). NOx is formed at the high temperatures encountered in the combustion flame and increase in formation as the combustion temperature increases. Exhausting NOx into the air has detrimental effects on the environment. NOx can photoreact and create air pollution and/or chemically react and contribute to acid rain. Accordingly, various systems have been developed to reduce NOx in the exhaust of internal combustion engines.
One such system is an exhaust-gas recirculation (EGR) system, wherein a portion of the exhaust gas is introduced into an air intake of the engine, thereby lowering the total oxygen concentration of the intake charge. Lowering the oxygen concentration decreases the peak combustion temperature, which, in turn, reduces the formation of NOx.
Although an EGR system reduces NOx formation, an EGR system increases hydrocarbon emissions. Moreover, only so much air can be replaced with the recycled exhaust gas before combustion becomes unstable.
Another system that is used to reduce NOx in exhaust gas is catalytic reduction, wherein NOx is removed from the exhaust gas by reaction with in-situ reductants, such as carbon monoxide or hydrocarbons (HC), when passed over a catalyst, typically containing rhodium. Such catalytic reduction, however, cannot cost-effectively reduce NOx to meet more stringent future government regulations.
Based on the foregoing, there is a need in the art for an improved method and apparatus for reducing oxides of nitrogen in the exhaust of an internal combustion engine. The present invention is directed to such a method and apparatus.
It therefore would be desirable, and is an advantage of the present invention, to provide an engine system having an internal combustion engine with a plurality of cylinders. Each of the cylinders has an intake port and an exhaust port. An air intake system is provided through which air is directed to the cylinders of the engine, and an exhaust system is provided for directing exhaust gas generated by combustion in the cylinders to the outside atmosphere. A pressurizing unit and a pressure swing adsorption unit are also provided. The pressure swing adsorption unit has an inlet connected to the pressurizing unit to receive compressed air therefrom, and an outlet connected to the air intake system of the engine. The pressure swing adsorption unit also includes an adsorbent bed cyclable between a production mode, wherein nitrogen in the air is adsorbed by the adsorbent bed at an elevated pressure to produce nitrogen-reduced air, and a regeneration mode, wherein nitrogen is desorbed from the adsorbent bed at a reduced pressure to produce nitrogen gas. The nitrogen-reduced air from the production mode of the adsorbent bed is supplied to the air intake system of the engine.
Also provided in accordance with the present invention is an air supply system for an internal combustion engine. The air supply system includes a pressurizing unit and a pressure swing adsorption unit, as described above. The air supply system further includes an air filter through which air may be directed to the pressurizing unit, and an air dryer connected between the air filter and the pressurizing unit. The air dryer contains a desiccant for removing water from the filtered air.
Also provided in accordance with the present invention is a method of reducing NOx in the exhaust of an internal combustion engine. The method includes the steps of providing an adsorbent bed and pressurizing air. The pressurized air is passed through the adsorbent bed, thereby raising the pressure of the adsorbent bed and causing the adsorbent bed to adsorb nitrogen from the air to produce nitrogen-reduced air. The nitrogen-reduced air is then directed into an air intake system of the engine. In another step, the pressure in the adsorbent bed is reduced such that nitrogen desorbs from the adsorbent bed. The nitrogen is then directed to the outside atmosphere.
Also provided in accordance with the present invention is an engine system including an internal combustion engine having an air intake system. Means for producing compressed air are provided. Also provided are means for producing nitrogen-reduced air and nitrogen gas from the compressed air using changes in pressure. The nitrogen-reduced air is provided to the air intake system of the engine.